Researchers said that “inefficient” food extraction was likely to mean poorer fitness among red squirrels, harming their chances of breedingGETTY IMAGES

They are smaller, weaker and more prone to disease, but perhaps the real reason they have disappeared is their lack of grey matter.

Native red squirrels are not as good at solving problems as invasive grey squirrels, a study has found.

Scientists studied how wild squirrels coped with two tasks to obtain hazelnuts — an easy one involving opening a transparent lid and a harder one in which they had to push and pull levers.

The two species were equally successful at the easy task but there was a clear difference for the difficult one, with 91 per cent of grey squirrels eventually solving it compared with 62 per cent of reds.

The researchers, from the universities of Exeter and Edinburgh, said the greys’ “superior…

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